1. Field of the Invention
This invention relates to dental composite materials and methods of manufacture thereof. In particular, this invention relates to improved glass fiber fillers for dental composite materials, wherein the glass fibers are densified and embrittled by heating the fibers.
2. Brief Discussion of the Related Art
Compositions useful for repairing damaged teeth in situ are known in the art as direct filling materials, and include alloys and resin composites. Dental amalgam alloys have widely been used as direct filling material, and provide excellent handling characteristics, and physical properties. The technique of mechanically packing and condensing a material into a tooth cavity is previously known to the dental profession in connection with the use of dental amalgams as a direct filling material. It has well known advantages in that it permits a close adaptation of the filling material to the cavity walls and also makes it possible to make firm contacts between the restored tooth and its neighbor. Further, it makes it possible to give the restoration its final anatomic form before hardening, thereby avoiding the time consuming and difficult finishing work with rotating instruments required with composite materials. These advantages are achieved by the densified, embrittled glass fiber composites on the present invention and the method of manufacture thereof. However, there are perceived health hazard concerns regarding the use of high amounts of mercury or gallium present in amalgam alloys.
Dental resins have accordingly been developed, which comprise polymeric matrices, for example polyamides, polyesters, acrylates, polyolefins, polyimides, polyarylates, polyurethanes, vinyl esters, or epoxy-based materials. Other polymeric matrices include styrenes, styrene acrylonitriles, acrylonitrile butadiene styrene polymers (xe2x80x9cABS polymersxe2x80x9d), polysulfones, polyacetals, polycarbonates, polyphenylene sulfides, and the like. The most popular polymeric matrices are based on monomers having at least one ethylenically unsaturated group, in particular acrylate and methacrylate groups. One commonly used monomer of this class is the reaction product of bisphenol A with glycidyl methacrylate (hereinafter BIS-GMA). In addition, these resins have also been used to make artificial teeth and denture basis.
Unfilled (i.e., pure) curable acrylic and methacrylic resins generally suffer from polymerization shrinkage and poor durability. These drawbacks have been reduced in direct filling applications, in part, through the addition of inert fillers. See, for example, U.S. Pat. No. 3,066,112 which is herein incorporated by reference. The combination of binder plus filler is commonly referred to as a composite direct filling material. Currently used fillers for curable dental resins generally are inert materials in the form of finely divided irregular particles, fibers or beads, present in an amount from about 35 to about 80 percent by weight of the total composite direct filling material.
Commonly used inorganic fillers include fumed silica, quartz, glass, various mineral silicates (e.g., xcex2-eucryptite, lepidolite, petalite, spondumene, beryl, topaz and zircon), silicon carbide, alumina, and mixtures thereof. Commonly-assigned U.S. Pat. No. 4,544,359 to Waknine, for example, discloses a filler mixture comprising barium silicate, borosilicate glass, and colloidal silica. In general composite direct filling materials which are fully loaded with inorganic fillers (i.e. combined with the highest workable volume loading) having particles in the range of 0.01-1.2 microns are the most wear-resistant currently available composite direct filling materials. However, these composite direct filing materials containing finely divided inorganic fillers and acrylic binder resins may not polish as easily as unfilled dental resin.
Organic materials have also been used as fillers. For example, U.S. Pat. No. 3,923,740 discloses a direct filling material containing finely divided cured polymethyl methacrylate, alone or in conjunction with an inorganic filler. Composite direct filling materials which are wholly or partly filled with finely divided polymethyl methacrylate have better polishability (i.e. better surface finish after polishing with ordinary dental tools) than composite direct filling materials which are fully loaded with inorganic fillers, but generally have poorer durability (i.e. poorer wear resistance in vivo) than composite direct filling materials having inorganic fillers only.
Regarding fibrous fillers in particular, U.S. Pat. No. 2,477,268 to Saffir discloses short glass fibers randomly dispersed in dental resin materials, as does U.S. Pat. No. 2,514,076 to Kelly. Use of long, fully wetted fibers in structural components for dental restorations and the like are disclosed in U.S. Pat. No. 4,894,012 to Goldberg et al. However, none of these patents is discloses a composite having the feel of amalgam.
Fused-fibrous filler compositions in the dental arts are also known. Such fused fibrous fillers are of particular interest because they reportedly provide a feel similar to that of amalgam when used by the dentist, and may be applied using similar techniques. In U.S. Pat. No. 4,381,918 and U.S. Pat. No. 4,392,828 to Ehrnford there is disclosed a filler comprising porous inorganic particles which are completely or partially impregnated with a resin material. The porous inorganic particles are formed by heating inorganic fibers under pressure to fuse the fibers at their points of contact, thereby forming a rigid three-dimensional network of inorganic fibers. Fused-fibrous fillers compositions are also disclosed in U.S. Pat. No. 5,621,035 to Lyles et al. Such fillers comprise silica fibers together with either alumina or aluminosilicate fibers which are fused in the presence of a fusion source such as boron nitride. The presence of boron lowers the melting point of the fibers sufficiently to allow formation of a porous, interconnected network. The network is then ground to particles having a size of about 180 microns, and used as fillers in dental composites.
Unfortunately, use of the fused-fibrous filler compositions disclosed in the Ehrnford and Lyles patents requires multiple steps and extensive preparation time. Accordingly, there is a need in the dental arts to develop a dental resin composite which is similar to or approaching to dental amalgam alloys in handling characteristics, physical properties, and applications without the drawbacks and deficiencies associated with dental amalgam alloys, and without the multiple preparation steps required for fused-fibrous compositions.
The above-described and other problems and deficiencies of the prior art are overcome or alleviated by the composition and method of manufacture of the present invention, comprising ground, densified, embrittled glass fiber fillers and a polymeric matrix. In accordance with the present invention, glass fibers are densified and embrittled by heating the glass fibers at an effective temperature substantially below the softening point of the glass fibers such that the glass fibers are densified and embrittled, which as used herein excludes fusing or melting together. The densified, embrittled fibers are then ground to a fibrous particle size preferably less than about 80 microns (xcexcm). The ground, densified, embrittled glass fibers of the present invention are preferably used as a filler component of a dental composite, being present in the range from about 5% to about 95% by weight of the total composition, depending upon the use made of the polymerized composition. In addition to the ground, densified, embrittled glass fibers, the polymeric matrix may further comprise other fillers known in the art.
The resulting dental composite exhibits handling characteristics similar to or approaching that of dental amalgam, and is particularly a suitable substitute filling for amalgam without the alleged health hazard concerns. The physical properties of the dental composite of the present invention include a modulus of elasticity of greater than approximately 15 GPa and modulus range between approximately 15 and approximately 22 GPa which is much higher than conventional dental composites which possess an elastic modulus of approximately 10 GPa. The elastic modulus of the composite of the present invention is comparable to amalgam which exhibits an elastic modulus of approximately 20 GPa. A relatively low modulus may cause a temporary displacement of the restoration resulting from normal masticatory stresses. Eventually this may cause leakage that may lead to secondary caries. Such movement or displacement is less likely to occur with materials having a high elastic modulus such as the composite of the present invention. Further, the composite exhibits a Vicker""s hardness of greater than 1000 MPa and preferably approximately 1,250 MPa enabling it to withstand stress better than conventional dental composites having a hardness in the range of 600 to 700 MPa. The wear rate is very low, averaging about 3.5 microns per year which is comparable to that exhibited by amalgam. The water sorption of the composite of the present invention is no greater than approximately 10 micrograms per cubic millimeter per week (xe2x80x9cxcexcg/mm3/wkxe2x80x9d) and preferably approximately 8.9 xcexcg/mm3/wk and polymerization shrinkage by volume is approximately 1.98%. The depth of cure of the composite is at least 5 mm which allows for bulk-fill application which is partly a result of the glass fibers used in the filler component. The fibers possibly serve as transparent pipes which transmit light to the bottom of the restoration. The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.